WASHINGTON — An Ohio congressman Tuesday accused the Reagan Administration of having ''screwed up'' budget priorities for opposing a $50 million addition to the Big Cypress National Preserve.

The $50 million would protect South Florida's water supply and endangered wildlife.

Rep. John Seiberling charged the administration with ''spending money on anything that involves the military'' at the expense of environmental programs. ''We cannot just go out of business because our priorities are all screwed up,'' he said.

Seiberling, a Democrat, made his remarks during a hearing on a proposal to buy 128,000 acres to the north and south of Alligator Alley, which is being converted into Interstate 75.

The road, which will run from Andytown to Naples, will cut the current access, which means the federal government has to pay severance fees to property owners. Sixty percent of the land is owned by two sides of the Collier family trust.

Under a bill sponsored by Sen. Lawton Chiles, D-Fla., and Rep. Tom Lewis, R-North Palm Beach, the $43.8 million in severance payments could be applied to the total $92.4 million value of land. The state would pay 20 percent of the $48.6 million difference, or about $10 million.

Buying the land would prevent more development, which already has strained the water supply and endangered wildlife in the area.

The measure's supporters, including the entire Florida congressional delegation and Gov. Bob Graham, have said the $48.6 million difference could be reduced further if the federal government were willing to swap other land parcels with the owners.

But during a hearing before the House Interior Committee's national park subcommittee Tuesday, an Interior Department official opposed the purchase and land swaps ''based on current program and budgetary priorities.''

''These lands do not appear to be essential for purposes of the existing Big Cypress National Preserve,'' said P. David Smith, deputy assistant secretary for Fish and Wildlife and Parks.

Lewis had argued that the land ''is worth preserving and enhancing for its unique and wild beauty, but it's also worth protecting because it forms the water supply system for 4.5 million South Florida residents.''

Graham called the proposal ''a rare windfall'' and said ''we will never have another chance to acquire this land so cheaply.''

But Smith estimated that with the federal-state cost split, the federal government would pay at least $40 million for the land.

Even if an owner donated land, ''substantial tax savings would likely result and the deficit would increase,'' Smith said. ''The deficit would also increase if federal properties, which are unneeded and could be sold directly, were exchanged.''

But after hearing testimony from Republican and Democratic lawmakers -- including Graham, Chiles, Lewis and Republican Reps. Bill McCollum of Altamonte Springs and Clay Shaw of Fort Lauderdale -- Sieberling zeroed in on Smith's agency.

He said former Interior Secretary James Watt's ''idelogical bent'' kept him from recognizing South Florida's serious problem of overpopulation and overdelveopment, and the critical need to protect the Biscayne Aquifer.

Graham said he was surprised by the Interior Department position, and said he hoped he and other Florida lawmakers could work out a solution with the administration.